Not unusual for random O365 blocks to appear, I especially like they way the reject message trys to refer the user back to their ISP as if their ISP is in any way involved with the internal Microsoft blocklist. “<<< 550 5.7.1 Unfortunately, messages from [x.x.x.x] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150).” If you log it here “https://olcsupport.office.com/“ , in a day or two they normally fix it, always after saying "I do not see anything offhand with the IP’s (x.x.x.x) that would be preventing your mail from reaching our customers", then you reply to them with “well there is a problem as you are bouncing the email as per the bounce message”. Then they fix it. Same routine every time. From: NANOG <nanog-bounces+tony=wicks.co.nz@nanog.org> On Behalf Of David Bass Sent: Thursday, January 11, 2024 7:32 AM To: <nanog@nanog.org> <nanog@nanog.org> Subject: Microsoft contact Hi everyone, hope y’all had a great holidays. I’m looking for a Microsoft Office 365 contact who can help us…we’re struggling to get anywhere using the standard methods. We have a customer whose subnet is blacklisted, and is causing a lot of heartache. We’ve proven to a couple of people at this point that Microsoft is blocking inbound traffic from this subnet, and so they can’t send/receive emails or access M365. This is a new eyeball network, so needless to say that it’s painful. Appreciate the help! David